1. Field of the Invention
The present invention relates to a communication system having an echo canceller for telecommunication lines and a line switching system.
The present application claims priority of Japanese Patent Application No. 2002-173259 filed on Jun. 13, 2002, which is hereby incorporated by reference.
2. Description of the Related Art
Conventionally, a communication system is available which includes a two-wire type transmission path and a four-wire type transmission path in a mixed manner, a two-wire/four-wire hybrid circuit (that is, a switching circuit between the two-wire type transmission path and the four-wire type transmission path, also being called “hybrid transformer”) to connect the two-wire type transmission path to the four-wire type transmission path, and an echo canceller (to be used for communication lines) to suppress an echo occurring due to diffraction of a signal in the two-wire/four wire hybrid circuit. Its typical example is a telephone line system or a like. Such a kind of the communication system, for example, employs both the two-wire type transmission path as a subscriber line to connect a plurality of telephone terminals to a switching device and a four-wire type transmission path as a central office line among switching devices.
Let it be here assumed that there is a case where, in the communication system as described above in which a two-wire type line and a four-wire type line exist in a mixed manner and which has a hybrid circuit and an echo canceller and, particularly, in a voice communication network of a line-switching type, a signal is transmitted between two points through both the two-wire type line and the four-wire type line and telephone speech is carried out via a communication connection (being also called “connection” or “call connection”) between communication terminal devices (that is, telephone sets) in particular. A path between the two points is set by a path setting process of a line switching processing section or of a switching device having a line switching function.
In the case of a transmission path (speech path) with a two-wire/four-wire hybrid circuit being connected in between, an echo is produced due to diffraction of a signal in the four-wire type transmission path and the two-wire/four-wire hybrid circuit. FIG. 5 shows a state in which the echo is produced in the transmission path with the two-wire/four-wire hybrid circuit 51 being connected in between. In the four-wire type transmission path 52, transmitting and receiving of signals are handled independently by using a separate line and, in the two-wire type transmission path 53, transmitting and receiving of signals are handled simultaneously by using a same line. As shown in FIG. 5, the two-wire type transmission path 53 is connected to the four-wire type transmission path 52 with the two-wire/four-wire hybrid circuit 51 being connected between them. A signal that passes through two lines installed on a lower side 52a (in the same figure) of four transmission path 52 is transmitted through the two-wire/four-wire hybrid circuit 51 to the two-wire type transmission path 53. A signal passing toward a direction of the two-wire/four-wire hybrid circuit 51 is transmitted through the two-wire/four-wire hybrid circuit 51 to the two lines installed on an upper side 52b (in the same figure) of the four-wire type transmission path 52 and reaches a receiver in the communication.
In the case of the transmission path with the two-wire/four-wire hybrid circuit 51 being connected between the two-wire type transmission path 53 and the four-wire type transmission path 52, an echo path is formed due to impedance mismatch in the two-wire/four-wire hybrid circuit 51. The echo is produced depending on a transmission characteristic of the echo path. As shown in FIG. 5, one part of the signal passing through the two lines installed on the lower side 52a of the four-wire type transmission path 52 reaches the two-wire type transmission path 53 through the two-wire/four-wire hybrid circuit 51, however, another part of the signal diffracts, in the two-wire/four-wire hybrid circuit 51, into a receiving signal passing through the two lines installed on the upper side 52b of the four-wire type transmission path 52. This diffraction causes formation of an echo path being turned back toward the four-wire type transmission path 52 (and toward communication terminals existing ahead) on the right in the same figure. An echo component caused by a transmitted signal (that is, transmitted speech) passing through the two lines installed on the lower side 52a of the four-wire type transmission path 52, together with a received signal (that is, received speech) passing through the two-wire type transmission path 53, reaches the two lines installed on the upper side 52b of the four-wire type transmission path 52.
In a front stage of the two-wire/four-wire hybrid circuit 51, if a transmission delay between the received signal and the transmitted signal is small, a time interval between the transmitted speech and an echo is short and, therefore, the echo is not recognized as a sound that can be heard at a communication terminal on a side of the four-wire type transmission path 52. However, if there is a large transmission delay between the received signal and the transmitted signal, the time interval between the transmitted speech and the echo is shifted, which causes the echo to be heard remarkably at the communication terminal and causes degradation in speech quality.
In order to suppress the echo occurring in the two-wire/four-wire hybrid circuit 51 existing on a speech path, an echo canceller 54 (to be used in communication circuits) is placed in the four-wire type transmission path 52 on the speech line, which cancels the echo passing from a direction of the two-wire/four-wire hybrid circuit 51. FIG. 5 shows configurations of the echo canceller 54 to be used in communication circuits. The echo canceller 54 shown in FIG. 5 performs training so as to learn a transmission characteristic of an echo path based on an input of a transmitting signal and to produce a pseudo-echo signal. By subtracting the produced pseudo-echo signal from a signal coming through the echo path from a direction of the two-wire/four-wire hybrid circuit 51, an echo component contained in a received signal passing through the two lines installed on an upper side 52b of the four-wire type transmission path 52 is suppressed and canceled. The echo canceller 54 is made up of an adaptive filter, subtractor, or a like (not shown).
However, the echo canceller 54 cancels an echo by learning a transmission characteristic of an echo path via training in a state in which the echo path exists and, therefore, time is required for completion of the training to learn the characteristic of the echo path before a training operation of the echo canceller 54 is converged, that is, the echo canceller 54 is put into a state in which it can cancel the echo. In a state existing immediately after connection is established to speak between communication terminals in a voice communication system (and immediately after an echo path is formed at a same time), that is, at time immediately after speech becomes enabled, training to learn the transmission characteristic of the echo path is not yet performed and an echo canceling function is not effective at this point. As a result, the echo is produced at an early stage of start of speech. As shown in FIG. 5, in the echo canceller 54 to be used in communication circuits, a transmitting signal passing through the two lines on the lower side 52a of the four-wire type transmission path 52 is used as an input and the training operation is converged based on the input.
As in the case of a method employed in Japanese Patent Application Laid-open No. Hei 3-188718, the method becomes effective in which the echo canceller performs training to learn the transmission characteristic of the echo path formed after establishment of connection by using the training signal fed to the echo canceller from a training signal source after connection has been established to smoothly cancel the echo.
In technology disclosed in the above patent application, the specified training signal source is placed on an input side of the echo canceller to be used in communication circuits on a four-wire type transmission path. After the establishment of connection, control is exerted so that the training signal is output to the echo canceller from the training signal source and so that the echo canceller performs a training operation based on the training signal and converges its training operation. After the training operation of the echo canceller has converged, speech is made possible in a state in which the echo canceling function is effectively achieved.
By employing such the training method to be performed by the echo canceller described above, training operations can promptly converge before actual speech by a telephone speaker is started, thus successfully suppressing the echo and improving speech quality. However, in the above communication system, connection between communication terminals (telephone sets) housed in a switching device is not limited to cases of origination (transmission of a signal) and termination (receipt of a signal) and, in some cases, connection has to be changed using a function of switching or a like in the switching device. The switching function represents a function of changing a path for calling connection by switching the path using a switch in the switching device, with calling connection between communication terminals being still maintained. When the path is changed, the echo path is also changed.
Since the technology disclosed in the above patent application does not provide a countermeasure against connection with the path being changed, the training operation to be performed on a newly formed echo path is not completed after the path has been changed and during time before the training operation is performed on the new path by the echo canceller and before the training operation converges, the echo is heard during telephone speech.
A first problem in the conventional technology is that, if a path is changed for signal transfer in connection between communication terminals, though the echo canceller performed a training operation and had an effective function of suppressing and canceling the echo at time of first establishment of connection, after the change of the path, it has no effective echo suppressing function during time before the training operation is completed and converged and, as a result, it is inevitable that the echo is heard at an early stage of start of speech following the change of the path.
This is because, in the conventional technology, the training operation is started based on a driving signal by the echo canceller only when the speech path is established for originating connection and terminating connection and, if the path is changed by a line switching function of the switching device with connection between communication terminals being maintained (that is, without being disconnected), an echo canceller's training operation to learn the transmission characteristic of a newly formed echo path is not performed.
A second problem is that, since, in the conventional technology, a driving signal is fed from the switching device to a training signal source connected to the four-wire type transmission path to cause the training signal to be supplied to the echo canceller for communication circuits, the control line used to control driving of the training signal for every echo canceller for communication circuits, in addition to the speech line, has to be prepared.
This is because, in the conventional technology, though a function of feeding the training signal to the echo canceller is controlled originally by the switching device, the feeding function is prepared as a separate function being different from presently-available functions (such as line switching functions, path changing functions and other various service functions) of the switching device.